GRAPH CONNECTIVITY. 9 Elementary Properties

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1 9 Elementary Properties 30 GRAPH CONNECTIVITY 9 Elementary Properties Definition 9.1: A graph G is said to be connected if for every pair of vertices there is a path joining them. The maximal connected subgraphs are called components. Definition 9.2: The connectivity number κ(g) is defined as the minimum number of vertices whose removal from G results in a disconnected graph or in the trivial graph (=a single vertex). A graph G is said to be k-connected if κ(g) k. Clearly, if G is k-connected then V (G) k + 1 and for n, m > 2, κ(k n ) = n 1, κ(c n ) = 2, κ(p n ) = 1 and κ(k n,m ) = min(m, n). Definition 9.3: The connectivity number λ(g) is defined as the minimum number of edges whose removal from G results in a disconnected graph or in the trivial graph (=a single vertex). A graph G is said to be k-edge-connected if λ(g) k. Theorem 9.1 (Whitney): Let G be an arbitrary graph, then κ(g) λ(g) δ(g). Proof: Let v be a vertex with d(v) = δ(g), then removing all edges incident to v disconnects v from the other vertices of G. Therefore, λ(g) δ(g). If λ(g) = 0 or 1, then κ(g) = λ(g). On the other hand, if λ(g) = k 2, let x 1 y 1, x 2 y 2,...,x k y k are be the edges whose removal causes G to be disconnected (where some of the x i, resp. y i, vertices might be identical). Denote V 1 and V 2 as the components of this disconnected graph. Then, either V 1 contains a vertex v different from x 1, x 2,...,x k, meaning that removing x 1,..., x k causes v to be disconnected from V 2. Or, V 1 = {x 1,...,x k }, where

4 10 Menger s Theorem 33 λ(g) = λ, by removing the edges between G 1 and G 2, and δ(g) = δ, by considering the vertex u δ+1. Suppose that condition 2 holds. Let G 1 = K κ, G 2 = K a, G 3 = K b and G 0 = G 1 + (G 2 G 3 ), where a = (n κ)/2 and b = (n 1 κ)/2 (notice, a+b = n κ 1) 7. To construct G, add a vertex v to G 0 and joint it to the vertices of G 1 and to δ κ vertices of G 3 (this is possible because 2δ + 2 n κ implies that δ κ b). Then, κ(g) = κ, by removing the vertices of G 1, λ(g) = λ, by removing the edges to v, and δ(g) = δ, by considering the vertex v. Finally, if condition 3, holds, set G = K n. 10 Menger s Theorem Definition 10.1: The local connectivity κ(x, y) of two non-adjacent vertices is the minimum number of vertices separating x from y. If x and y are adjacent vertices, their local connectivity is defined as κ H (x, y) + 1 where H = G xy. Similarly, we define the local edge-connectivity λ(x, y). Clearly, κ(g) = min{κ(x, y) x, y G, x y}. The aim of this section is to discuss the fundamental connections between κ(x, y) and the set of xy paths. Two paths in a graph G are said to be independent if every common vertex is an endvertex of both paths. A set of independent xy paths is a set of paths any two of which are independent. Obviously, if there are k independent xy paths then κ(x, y) k. Menger s Theorem states that the converse is true. We prove the theorem by means of an elegant proof by Dirac (1969). Theorem 10.1 (Menger (1926)): Let x, y G, x y. There exists a set of κ(x, y) independent paths between x and y and this set is maximal. Proof: We use induction on m = n + e, the sum of the number of vertices and edges in G. We show that if S = {w 1, w 2,..., w k } is a minimum set (that is, a subset of the smallest size) that separates x and y, then G has at least k independent paths between x and y. The case for k = 1 is clear, and this takes care of the small values of m, required for the induction. (1) Assume that x and y have a common neighbor z Γ(x) Γ(y). Then necessarily z S. In the smaller graph G z the set S z is a minimum 7 G+H is used here to reflect the graph obtained by G H and adding an edge between every vertex x G and y H

5 10 Menger s Theorem 34 set that separates x and y, and so the induction hypothesis yields that there are k 1 independent paths between x and y in G z. Together with the path xzy, there are k independent paths in G as required. (2) Assume that Γ(x) Γ(y) = and denote by H x and H y as the connected components of G S for x and y, respectively. (2a) Suppose that the separating set S Γ(x) and S Γ(y). Let z be a new vertex, and define G z to be the graph with the vertices V (H x S z) having the edges of G[H x S] together with the edges zw i for all i = 1,...,k. The graph G z is connected and it is smaller than G. Indeed, in order for S to be a minimum separating set, all w i vertices have to be adjacent to some vertex in H y. This shows that e(g z ) e(g) and, moreover, assumption (2a) rules out the case H y = y, therefore n(g z ) < n(g) in the present case. If T is any set that separates x and z in G z, then T will separate x from all w i S (T S) in G. This means that T separates x and y in G. Since k is the size of a minimum separating set, T = k. We noted that G z is smaller than G, and thus by the induction hypothesis, there are k independent paths from x to z in G z. This is possible only if there exist k independent paths from x to w i, for i = 1,..., k, in H x. Using a symmetric argument one finds k independent paths from y to w i in H y. Combining these paths proves the theorem. (2b) Suppose that all separating sets S are a subset of Γ(x) or Γ(y). Let P be the shortest path from x to y in G, then P contains at least 4 vertices, we refer to the second and third node as u and v. Define G n as G uv (that is, remove the edge between u and v). If the smallest set T that separates x from y in G n has a size k, then by induction, we are done. Suppose that T < k, then x and y are still connected in G T and every path from x to y in G T necessarily travels along the edge uv. Therefore, u, v T. Also, T u = T u and T v = T v are both minimum separating sets in G (of size k). Thus, T v Γ(x) or T v Γ(y) (by (2b)). Now, P is the shortest path, so v Γ(x), hence, T v Γ(y). Moreover, u Γ(x), thus T u Γ(x). Combining these two results we find T Γ(x) Γ(y) (and T is not empty). Which contradicts assumption (2). The set is maximal, because the existence of k independent paths between x and y implies that κ(x, y) k. Another way to state this result is the following: A necessary and sufficient condition for a graph to be k-connected is that any two distinct vertices x and y can be joined by k independent paths.

6 11 Additional Exercises 35 Exercises 10.1: On Menger s Theorem: Let G be a graph with G k +1, then G is k-connected if and only if for all k-element subsets V 1, V 2 V (G), there is a set of k paths from V 1 to V 2 which have no vertex in common. [V 1 V 2 is not necessarily empty, thus, some paths might be trivial paths]. Let U be a set of vertices of a graph G and let x be a vertex not in U. An xu fan is defined as a set of U paths from x to U, any two of which have only the vertex x in common. Theorem 10.2 (Dirac (1960)): A graph G is k-connected if and only if G k + 1 and for any k-set U V (G) and x V (G) U, there is an xu fan. Proof: (a) Suppose that G is k-connected, U V (G), U = k and x V (G) U. Let H be the graph obtained from G by adding a vertex y and joining y to every vertex in U. Clearly, H is also k-connected. Therefore, by Menger s theorem, we find that there are k independent xy paths in H. Omitting the edges incident with y, we find the required xu fan. (b) Suppose G k + 1 and that S is a (k 1)-set separating x and y, for some vertices x and y. Then, G does not contain an x(s y) fan. Exercises 10.2: On Dirac s Theorem: 1. If G is k-connected (k 2), then for any set of k vertices {a 1,...,a k } there is a cycle containing all of them. [Hint: Use induction on k and distinguish between the case where the cycle C contains an additional vertex x {a 1,..., a k 1 } and the case where it does not.] 2. Give an example of a graph for which there is for any set of k points, a cycle containing all of them, but that is not k-connected (k > 2). 11 Additional Exercises Definition 11.1: Let k 1. Consider the set B k of all binary sequences of length k. For instance, B 3 = {000; 001; 010; 100; 011; 101; 110; 111}. Let Q k be the graph (called the k-cube) with V (Q k ) = B k, where uv E(Q k ) if and only if the sequences u and v differ in exactly one place.

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